Sonic-color system

ABSTRACT

An electronically controlled audio-responsive color display is provided by a system wherein audio signals of various frequencies are utilized to effect illumination in various colors. Signal processing amplifies and limits total audio signal strength while substantially maintaining relative signal strength. The audio signals are segregated into a plurality of frequency bands, and the detected signal magnitude from each band is applied in pulse form to effect illumination in corresponding colors.

United States Patent 1 1 McClure June 4, 1974 1 SONIC-COLOR SYSTEM Primary ExaminerHarold l. Pitts 76] Inventor: Robert Bruce McClure, RD. 2 Box NO 290, Malvem 19355 Attorney, Agent, or Fzrm-Charles A. McClure F' O t. 12 1971 [22] c 57 ABSTRACT .N 3 9 [21] Appl O 3 An electronically controlled audio-responsive color display is provided by a system wherein audio signals H 340/366 B, 340/148 R, 340/261 R of various frequencies are utilized to effect illumina- [51] Int. Cl. G081) 5/22 tion in various colors Signal processing amplifies and 1 1 Field 0f ch 340/148 R, 261 R, 6 B limits total audio signal strength while substantially maintaining relative signal strength. The audio signals [56] e erenc Cited are segregated into a plurality of frequency bands, and

UNITED STATES PATENTS the detected signal magnitude from each band is ap- 2910.681 10/1959 Milli 340/324 R plied in Pulse form to effect illuminatiO" in 3.o3s,061 6/1962 OReilly 340/261 ux Spending Colors- 3,234,847 2/1966 Williams 340/261 UX 3.579.187 5/1971 Knott 340/148 R 13 Claims, 13 Drawing Figures PATENTEDJuu 4:914

SHEET 10F LINE DLSPLAY POWER DETECT F\LTER AGC AUDlO. \NPUT m/v /vm? Y ROBERT BRUCE McCLUEE PATENTEBJUH 4:914 3.815; 128

- IM/EATOR ROBERT BRUCE McCLURE SONIC-COLOR SYSTEM This invention relates to production of color displays derived from signals of audio or sonic frequency and provides novel methods and apparatus for doing so, collectively referred to herein as a sonic-color system.

It is known to produce light displays in various colors dependent upon an input of audio-frequency signals derived from the output stage of a radio receiver, record player, tape recorder-reproducer, or the like. Interesting illumination effects are obtainable when the colors thereof are coordinated in some way with the sonic or audio frequencies. However, existing systems for doing so are deficient in a number of respects, including frequency separation, response control, and vi sual effect.

A primary object of the present invention is a soniccolor system having improved response to input audiofrequency signals.

Another object is a multiple-channel sonic-color system having excellent, separation between channels.

A further object is a sonic-color system having novel display characteristics.

Other objectsof this invention, together with means and methods for attaining the various objects, will be apparent from the following description and the accompanying diagrams thereof shown by way of example rather than limitation.

FIG. I is a block diagram of components of the invention whether considered as method or apparatus;

FIG. 2 is a schematic diagram of an apparatus component corresponding to the first (extreme left) block of FIG. I;

FIG. 3 is a schematic diagram of an apparatus component useful according to the second block of FIG. 1;

FIG. 4 is a circuit diagram of an apparatus component useful according to the third (middle) block of FIG. 1;

FIG. 5 is a schematic diagram of an apparatus domponent useful according to the fourth block of FIG. 1;

FIG. 6 is a schematic diagram of an apparatus component usefulaccording to the fifth (extreme right) block of FIG. 1;

FIG. 7 is a perspective view of a display screen useful in the component of FIG. 6; and

FIG. 8 is a fragmentary sectional plan view, taken at VIIIVIII on FIG. 7.

FIG. 9 is a circuit diagram of the component shown more schematically in FIG. 2;

FIG. 10 is a circuit diagram of part of a low-pass component corresponding to that shown more schematically in FIG. 3;

FIG. 11 is a circuit diagram of part of an intermediate-band component corresponding to that shown more schematically in FIG. 3;

FIG. 12 is a circuit diagram of part of a high-pass component corresponding to that shown more schematically in FIG. 3; and

FIG. 13 is a circuit diagram of the component shown more schematically in FIG. 5.

In general, the objects of the present invention are accomplished, in an electronically controlled audioresponsive color display system, wherein audio signals of various frequencies are utilized to effect illumination in various colors, by the method combination of amplifying the signals and limiting the total audio signal strength to a desired level while substantially maintaining relative signal strength, segregating the signals into a plurality of frequency bands, then for each band detecting the signals therein, producing pulses whose duration is dependent upon the detected signal magnitude for such band, and applying the pulses from the respective bands to effect illumination in corresponding colors.

In its apparatus aspect this invention contemplates means for accomplishing such method in conjunction with chromophoric display means responsive to such pulses to be illuminated in colors corresponding to the respective bands.

FIG. 1 shows that audio signals from any suitable source, such as aforementioned in connection with existing sonic-color systems, is subjected to automatic gain control (AGC), whereupon the system acts to Filter, Detect, Power, and Display the signals as successively acted upon by the method steps and apparatus components of this invention. The method is described,

and the apparatus shown, in more detail'as follows.

FIG. 2 shows the AGC component schematically as made up of operational amplifier (op-amp)v A having input impedance Z, through which the audio input is applied from input terminal a to the upper left side or inverting input terminal of the amplifier, with the lower left side or non-inverting terminal grounded. The amplifier output appears at the right or apex terminal of the amplifier, which connects via feedback impedance Z to the input terminal and also connects via output terminal b to the component shown in the next view. Details of a preferred embodiment of the component of the present view appear in FIG. 9, considered hereinafter.

- FIG. 3 shows schematically a single channel of active filter means having input terminal b corresponding to the output terminal of FIG. 2. The illustrated circuit constitutes one of a plurality of such channels: viz., a low-pass channel, a high-pass channel, and one or more (preferably three) intermediate-band channels. Each channel comprises a pair of operational amplifiers, designated generally as A,- and A}, together with appropriate input and feedback impedance networks: Z through Z for A, and Z through Z for A, for which corresponding circuit elements appear in FIG. 10 (lowpass), FIG. 11 (intermediate or bandpass), and FIG.

l2 (high-pass). Each amplifier is like that of FIG. 2, as

outlined in broken lines in FIG. 9. The amplifier stages are connected in cascade, and the circuit elements providing impedances Z, and 2,, are preferably adjustable to facilitate balancing of the sensitivity or gain of the cascaded stages and to assure the desired response. As with the amplifier in the preceding view, the noninverting input (each stage) is grounded. The respective channels have separate output terminals, designated generally by 'c in this view, to corresponding detectors (one of which is shown in the next view).

FIG. 4 shows the circuit of a detector stage for a representative one of the filter channels. Diode D is connected in the forward direction from input terminal c to output terminal d while capacitor C is connected between the output terminal and ground. The output (now d.c.) passes to a power control channel such as is shown in the next view.

FIG. 5 shows schematically that last component before the display stage. Input impedance Z is connected between terminal d and ramp generator G. Also connected to the generator input is synchronizer S, itself connected to ac. terminals f, g suchas a conventional power line or equivalent source. The same synchronizer is useful in synchronizing the ramp generators of a plurality of like power control channels, one per filter channel, and optionally one or more others to control one or more additional display features. Accordingly, the block for synchronizer S is surrounded by broken lines as a reminder of its status distinct from an individual power control channel. Output impedance Z is connected between its ramp generator and triac stage T, which has output terminals e,f to display means (shown in the next view) and one side (grounded) of the ac. source. Circuit details for the apparatus of this view appear in FIG. 13. I

FIG. 6 shows the display means schematically, including plurality of lamps X,. X interconnected between terminal 12 (FIG. and ac. source terminal g.

The lamps are adjacent a display screen comprising a plurality of portions Y Y,,, at least one (and preferably more than one) such-portion per lamp. Shown are only the extreme or end members of the series of lamps and of screen portions. Intervening parts are broken away to simplify the illustration. Either or both the lamps and the screen are colored appropriately,.and

opaque spacers (not shown) are useful: between lamps of diverse colors and adjacent to the divisions between diversely colored screen portions. The screen, which'is shaded to indicate plastic composition, may be quite simple in design, as suggested in this view, or more elaborate, as in the next view.

FIG. 7 shows in perspective, on a greatly reduced scale, color display screen Y comprising rectilinear mosaic-like colored portions (so shaded) and frame F. The rear face (not shown) is a mirror image of the visible front face. Spacer means I, which intervenes between adjacent screen portions (of different colors) apears more clearly in FIG. 8, which shows a horizontal section therethrough and through the intervening spacer means and the frame.

Operation of the disclosed apparatus to practice the disclosed process is readily understood. Of course, al-

though not shown, suitable d.c. sources are used to provide positive and negative potentials to various of the apparatus components.

Audio-frequency signals at the AGC input are amplified and the total audio signal strength is limited by suitable gain control adjustment to a desired level in that component (FIGS. 2 and 9) without substantially disturbing relative signal strength. The amplified audio output from the AGC component is fed to each (FIG. 3) in a bank of frequency filters, at least three and preferably five in number, including a low-pass, a high-pass, and one or more (preferably three) intermediate bandpass channels. These filters are active, rather than passive, and include two operational amplifiers (each like that outlined in broken lines in FIG. 9) with input and feedback networks therefor (FIGS. 10, 11, and 12 for the low-pass, band-pass, and high-pass channels, respectively), which are connected in cascade. The interdc. signals are fed through high input impedance to respective power control channels, FIGS. 5, 13, one per filter channel. The ramp generator is synchronized to the ac. source frequency by synchronizer S, which conveniently controls several power control channels similarly. The input d.c. signal from the detector stage determines the decay of the ramp. The triac receives through the output impedance network a succession of pulses whose duration, i.e., the summed duration of individual pulses also corresponds to the input signal, and such pulses control conduction through the triac and, thus, through the display means, which is connected to the triac output and thereby across the ac. source (one side of which is grounded). In this way the channels are isolated effectively from one another, and the brightness response of the display means accurately reflects relative band strength. Controls for brightness and threshold provide a full range of response so as to minimize full-on and full-off periods.

It will be understood that each band effects illumination in a different color, preferably ranging from low to high frequency of color in accordance with low to high frequency of sound, thus: red for low-pass; orange, yellow, and green for the successively higher band-pass;

and blue for the high-pass. It is preferable that a plurality of portions of the display screen be illuminated in each of the given colors and that the total area or apparent area of the portions for each color be the same as for every other one of the colors. The prepared color display screen of FIGS. 6 and 7 has these attributes and also aspects of symmetry and other esthetically desirable attributes, and its design is the subject of my design patent application, Ser. No. D-l6l,37l filed July 12, 1971 and now Pat. 226,777 Although shown herein in the form of transparent plastic composition (e.g., methyl methacrylate or other acrylic or'similar material, such as that called Plexiglas' and available mediate or band-pass op-am ps, FIG. 11, are staggertuned (preferably approximately 0.7 octave) to square from Rohm & Haas Co., Philadelphia) for use with adjacent lamps actuated by the described electronic apparatus, the screen may comprise suitable chromophoric (e.g., phosphor) materials. if desired, and be actuated directly by such apparatus.

The electronic apparatus of this invention is suited for use with any low-impedance source of audiofrequency signals within the range of 0.1 to 5 volts peak-to-peak (pp) without objectionable loading of the source. It is adapted to produce an output of up to 500 wattsper channel. Advantages and benefits of its structural and functional features have been mentioned above and, together with others, will become apparent and accrue to persons undertaking to practice this invention in the light of the foregoing disclosure and the following more detailed consideration of the apparatus compon nts. I

FIG. 9 shows the AGC circuit, which is provided with two input terminals a,',a (rather than simply a as in FIG. 2) to accommodate. stereo input, ifdesired. With input from 8 ohm speakers a separation of db ismaintained. Input resistors R and R (4.7 k each) lead from the respective input terminals to the junction of resistor R (560 ohms) and electrolytic capacitor C (5 uf). The other'side of C connects to the junction of R (2.2 Meg) and R l Meg), which together form a voltage divider from ground to positive l5 V) dc. potential. v

lnterposed between the AGC op-amp is-MOSFET (metal oxide semiconductor field effect transistor) Q,

and associated circuit elements. its source (S) and emitter (E) electrodes are. tied to the junction of R and R just mentioned, and its drain electrode D connects thereto through feedback resistor R (470 k) and to the non-inverting input terminals of the op-amp through d.c. blocking electrolytic capacitor C (5 uf). its gate electrode receives positive feedback from a further stage after the op-amp.

The AGC op-amp is conveniently of 709C type and includes (outlined in broken lines) associated circuit elements: first capacitor C pf) connected across terminals 5 and 6 (output), second capacitor C" (500 pf) connected in series with resistor R (1.5 k) across terminals 1 and 8. Terminals 4 and 7 go to sources of and respectively. Terminals 2 and 3 are the I inverting and non-inverting input terminals, respectively, the latter of which is grounded. The same operational amplifier circuitry (and values of circuit elements) is used in both stages of each filter channel with appropriate feedback networks. The AGC op-amp has respective fixed and adjustable feedback resistors R (33 k) and R (0.75 Meg) from output terminal 6 to input terminal 2. The setting of adjustable resistor R pled through R and C to pulse ()5 to its nonconducting state, thereby causing a positive peak detected bias to the Q, gate, whereupon the resulting increase in source-to-drain resistance reduces the overall gain, which is adjustable by the setting of R as the master gain control. The audio signal so amplified and limited (voltage gain of about ten times) is applied through electrolytic d.c. blocking capacitor C to the diverse filter channels at terminal b.

As already mentioned, the filter channels (shown generically in FIG. 3) each comprise a pair of op-amps A, and A, (like A of the AGC component), each with its own feedback network and associated circuit elements. The feedback circuit elements are identified in FIG. 10, 1 1. and 12 for the low-pass, intermediate or band-pass, and high-pass channels, respectively, using the same subscripts as in the impedance blocks of FIG. 3. Prefixed are added subscript L for low-pass. primed (singly, double. and triply) for the band-pass. and with added subscript H for the high-pass. It will be understood that the circuitry intersections designated as r, s.

determines the maximum gain state. voltage gain, t, and u in FIGS.- 10, 11, and 12 guide the substitution thereby determiningat what input signal level the outof their circuits at correspondingly designated intersecput comes under control. tions in the respective stages (with subscriptdesigna- The further stage in the AGC component comprises tions i and j) in accordance with FIG. 3. The circuit eletransistor 0 (2N51 35 type) with its emitter electrode ments making up impedances Z and 2,, (FIG. 3) are grounded and its collector and base electrodes supplied added within a block shown in broken lines and so deswith' positive (15 v) d .c. potential through resistors R ignated in FIG. 12 for the high-pass channel portion. I.l k) and R (220 K) respectively. The op-amp out- I Appropriate values for these and the associated eleput is injected at the 0 base through adjustable resistor ments of the filter channels appear in the following ta- R k) and electrolytic capacitor C 10 ,uf) co n; b l e TABLE I Filter channel Low-pass (6db Band-pass, (mid- Band-pass; (midgand-pass gid- High-pass (6db impedance block down at 200 Hz) freq. 320 Hz) freq. 640 Hz) freq. 1300 Hz) down at 2000 Hz) R =25k z, (all adjustable) Ru=25k R '=25k R "=25k' R"'=25k R,,,= 10k (adj.) Cm z C,,2=0.15 r R =4.5k R2"=210k R k R =33k z, R,, =4.7k, c." =0.o1,tr c,"=o.o1,tr c."'=0.0| r c,,,=o.o02s r Z4 R =56k C '=0.0luf C "=0.0luf C4"'=.0.0lp.f C =l25pf 2,, CI, 0.0075uf R5 800k RB 260k R5 k R".-,= 820k R =2s| zH (all adjustable) Rit=25k t =2 t {RZ:,,= 10k (adj) z, c,.1=o.1s,tr R1=3.8k R1=2-3k R1"'=450n f z R,, =4,.7k Cg'=o.0l}l.f C "=0.0luf C.."'=0.0i f C,,,=0.002s r 2., R,,.,=56k C =0.0lp.f Q"=0.0iaf C,,"'=0.0luf Cm'.= l25pf z,u ig. =4 03 Rw 140k Rm= 820k nected in series. The O collector connects to Q gate 60 The output from each filter channel is fed to a correelectrode G through diode D (lN98 type). Connected between the diode lead and ground on the transistor side is resistor R 15 K), and on the MOSFET side R (1.8 Meg) and (in parallel) electrolytic capacitor C normallysaturated by reason of the positive bias of its base through R This occasions a minimum bias potential (ca. 0.3 v) applied to the Q, gate, whereupon sponding detector component (FIG. 4) in which the diodes are of 1N98 type and the capacitors have the following values:

Of course, in each instance the detector output is d.c., varying in accordance with the channel or band signal strength. Such output is used as a control potential in the respective power control units, as shown in the final view,

FIG. 13 shows the circuitry for one of the power control units together with that of a synchronizer (S, outlined in broken lines, as in FIG. useful with more than one such unit. Detector potential from d through resistor R (4.3 k) is furnished to the input elements,

connectors are tied together and whose emitters are grounded, constitute together with the mentioned .resistors a double-ended expander (e.g., Motorola MC 785 P type). Resistor R16 (680 ohms) is connected from the positive d.c. potential source to the common connector terminal of the synchronizer and also to a similar but single-ended expander stage through its b esis o 15.(4 h s) The synchronizer, operating through Q3, determines the input impedance totransistor Q4 of the ramp generator by alternately saturating Q and O except for recurrent brief (e.g., 0.5 msec) periods. When the applied a.c. potential is in the vicinity of zero (less than about 0.5 v), then 0 receives a saturating positive pulse. At each such recurrence the ramp generator is reset by the charging of capacitor C which parallels base resistor R (450 ohms), through collector resistor R (640 ohms). Between such recurrences C discharges through Q and R at a rate dependent upon the input control potential from the detector, the rate of discharge increasing with such positive potential. Both 0, with its associated resistors and the following stage, 0 with corresponding base and collector resistors R (450 ohms) and R (640 ohms), are inverters (e.g., Motorola MC .789 P type).

Reduction of the Q, collector potential below about 0.6 v cuts off Q which otherwise is saturated. This applies a positive pulse to the gate electrode of triac Q1 larger the input the longer the lamps are actuated as well as the brighter they are.

A sixth power control unit is useful to actuate other display features, such as background lighting (preferably inversely) or to superimpose other effects (e.g.. moire) upon an illuminated display screen. When six such units are used, it is convenient to use two synchronizers, synchronizing three units each.

in triac stage T through an emitter follower stage 6, are actuated from the ac. potential'source.

. Experience has shown that brightness change in.

lamps of the usual Christmas-tree variety, which may be used in such display means if desired, is apparent over a range of about 0.5 to 2.3 v. Below the lower value no visible light is apparent, and R can be adjusted to eliminate that portion of the response curve' by adding compensating positive bias. The responsive is quite linear over the visible rangeand, of course, the

Modifications of the present invention, such as by addition, combination, or subdivision of parts or steps, or substitution of equivalents, may be made while retaining significant advantages and benefits of the invention. which is defined in the following claims.

The claimed invention:

1. In an electronically controlled audio-responsive color display system, wherein audio signals of various frequencies are utilized to effect illumination in various colors, the method combination of amplifying the signals and limiting the total audio signal strength to a desired level while substantially maintaining relative signal strength, segregating the signals into a plurality of frequency bands, then for each band detecting the signals therein, producing periodic pulses of substantially constant amplitude whose duration is dependent upon the detected signal magnitude for such band, and applying the pulses from the respective bands to effect illumination in corresponding colors.

2. Color display system according to claim I, wherein the color intensity varies in accordance with the pulse duration and, thus, the detected signal magnitude.

3. Color display system according to claim I, including the step of synchronizing the pulses.

4. Color display system according to claim 1, wherein the signals are segregated into five bands, including a low-pass band and a high-pass band, and also including three intermediate-frequency bands each substantially one octave wide.

, 5. Color display system according to claim 4, wherein the effected illumination has the following colors, from the lowest to highest band frequency; red, orange, yellow, green, and blue.

6. In an electronically controlled audio-responsive color display system, the apparatus combination of automatic gain control means (AGC) for amplifying input audio-frequency signals from a source thereof and limiting total audio signal strength to a desired level while substantially maintaining relative signal strength, filter means interconnected thereto for segregating the signals into a plurality of frequency bands, separate detector means for each band connected to receive the sigsource and the amplifier input and comprising fieldeffect transistor means having a gate electrode connected to the feedback network.

8. Color display system according to claim 6, wherein the filter means comprises a plurality of operational amplifiers connected cascade and stagger-tuned to pass respective adjacent bands of frequencies.

9. Color display means according to claim 6, wherein the pulse-producing means comprises ramp-generating means, means synchronizing the ramp-generating means to an a.c. source, and triac means interposed between the a.c. source and the display means and having a gate electrode interconnected to the output of the ramp-generating means.

10. Color display means according to claim 6, including a plurality of light-emitting means corresponding to the plurality of bands, and a plurality of diverse colored light-transmitting means located for illumination by the respective emitting means.

11. In an electronically controlled audio-responsive color display system operated from a.c. supply potential including means for filtering audio-frequency signals from a source thereof into several bands, means for detecting separately the signals in the respective bands, and means for actuating chromophoric display means in various colors corresponding to the respective bands in accordance with the detected signals, the improvement wherein the actuating means includes pulseproducing means associated with the respective bands and effective to produce series of pulses whose duration, as distinguished from their amplitude, is dependent upon detected signal magnitude, and output means gated thereby to conduct current to the chromophoric display means for periods corresponding to the pulse duration and otherwise to be non-conducting.

12. Color display system according to claim 1, wherein control of pulse duration is effected by varying the values of phase angle in an a.c. supply potential at which pulse production is initiated.

13. Color display system according to claim 12, wherein the pulses of resulting varying duration are applied to control output power in proportion to their duration and, therefore, proportional to the amplitude of the corresponding filtered signals. 

1. In an electronically controlled audio-responsive color display system, wherein audio signals of various frequencies are utilized to effect illumination in various colors, the method combination of amplifying the signals and limiting the total audio signal strength to a desired level while substantially maintaining relative signal strength, segregating the signals into a plurality of frequency bands, then for each band detecting the signals therein, producing periodic pulses of substantially constant amplitude whose duration is dependent upon the detected signal magnitude for such band, and applying the pulses from the respective bands to effect illumination in corresponding colors.
 2. Color display system according to claim 1, wherein the color intensity varies in accordance with the pulse duration and, thus, the detected signal magnitude.
 3. Color display system according to claim 1, including the step of synchronizing the pulses.
 4. Color display system according to claim 1, wherein the signals are segregated into five bands, including a low-pass band and a high-pass band, and also including three intermediate-frequency bands each substantially one octave wide.
 5. Color display system according to claim 4, wherein the effected illumination has the following colors, from the lowest to highest band frequency; red, orange, yellow, green, and blue.
 6. In an electronically controlled audio-responsive color display system, the apparatus combination of automatic gain control means (AGC) for amplifying input audio-frequency signals from a source thereof and limiting total audio signal strength to a desired level while substantially maintaining relative signal strength, filter means interconnected thereto for segregating the signals into a plurality of frequency bands, separate detector means for each band connected to receive the signals therefrom, pulse means associated with the respective detector means and adapted to produce series of pulses of substantially constant amplitude whose duration is dependent upon detected signal magnitude, and connected chromophoric display means responsive to such pulses to be illuminated in colors corresponding to the respective bands.
 7. Color display means according to claim 6, wherein the AGC means comprises an operational amplifier stage having an output feedback network, and variable-impedance means interconnected between the audio source and the amplifier input and comprising field-effect transistor means having a gate electrode connected to the feedback network.
 8. Color display system according to claim 6, wherein the filter means comprises a plurality of operational amplifiers connected in cascade and stagger-tuned to pass respective adjacent bands of frequencies.
 9. Color display means according to claim 6, wherein the pulse-producing means comprises ramp-generating means, means synchronizing the ramp-generating means to an a.c. source, and triac means interposed between the a.c. source and the display means and having a gate electrode interconnected to the output of the ramp-generating means.
 10. Color display means according to claim 6, including a plurality of light-emitting means corresponding to the plurality of bands, and a plurality of diverse colored light-transmitting means located for ilLumination by the respective emitting means.
 11. In an electronically controlled audio-responsive color display system operated from a.c. supply potential including means for filtering audio-frequency signals from a source thereof into several bands, means for detecting separately the signals in the respective bands, and means for actuating chromophoric display means in various colors corresponding to the respective bands in accordance with the detected signals, the improvement wherein the actuating means includes pulse-producing means associated with the respective bands and effective to produce series of pulses whose duration, as distinguished from their amplitude, is dependent upon detected signal magnitude, and output means gated thereby to conduct current to the chromophoric display means for periods corresponding to the pulse duration and otherwise to be non-conducting.
 12. Color display system according to claim 1, wherein control of pulse duration is effected by varying the values of phase angle in an a.c. supply potential at which pulse production is initiated.
 13. Color display system according to claim 12, wherein the pulses of resulting varying duration are applied to control output power in proportion to their duration and, therefore, proportional to the amplitude of the corresponding filtered signals. 